If you’re a developer interested in building for the Google+ platform,
let us know and we’ll contact you when we have something for you to
preview. Just remember, like the rest of the Google+ project this is
only the beginning, so stay tuned.

Deducing from the existing games on Google+ Games it seems that you start creating one using Flash or JavaScript. I believe anything that works on modern browsers should also work on Google+ with little tweak. I'm not waiting for API to come out to develop a game. I think API would merely open up the pipeline to pull/push data onto Google+ platform.